Network switches play a critical role in large computer networks, such as those found in a data center. Server computers in the network can be divided into racks, and network switches can connect the server computers within a rack to routers in the data center. Data passed by switches is generally divided into a series of packets that can be transmitted between devices. Packets include control information and payload data. The control information includes information used to deliver the payload data. For example, control information can include source and destination network addresses, error detection codes, packet sequencing identification, and the like. Typically, control information is found in packet headers and trailers included within the packet and adjacent to the payload data.
Generally, network switches have two primary planes: a control plane and a data plane. The control plane is a management plane that configures the data plane. The data plane receives packets on input ports and transmits the received packets to output ports based on the configuration. A forwarding table within the data plane controls which next hops receive the packets. Typically, the forwarding table is programmed in accordance with routing strategies, such as equal-cost multi-path (ECMP) routing, which is a strategy where next-hop packet forwarding to a destination can occur over multiple best paths. ECMP is per-hop decision limited to a single router, and can be inefficient if routers in the network are not functioning.
Testing of network switches can be problematic. Typically, test probes are sent from a source host device through a network of switches to a destination host device. The probes are effective at testing the paths and links in the network, but not effective at testing forwarding states of the network devices themselves.